Method of making high-speed, low-noise millimeterwave HEMT and pseudormorphic HEMT

ABSTRACT

An epitaxial structure and method of manufacture for a field-effect transistor capable of high-speed low-noise microwave, submillimeterwave and millimeterwave applications. Preferably, the epitaxial structure includes a donor layer and/or buffer layer made from a semiconductor material having the formula AlP 0 .39+y Sb 0 .61-y.

This is a division of application Ser. No. 08/466,156 filed Jun. 6,1995, now U.S. Pat. No. 5,548,140.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to the art of electronictransistors and more specifically, to a novel material system andepitaxial structure for a modulation-doped field-effect transistor(MODFET) or lattice-matched and pseudomorphic high electron mobilitytransistors (HEMTs and pHEMTs).

2. Description of Related Art

A MODFET is a field-effect semiconductor transistor designed to allowelectron flow to occur in an undoped channel layer so that the electronmobility is not limited by impurity scattering. MODFETs are used in avariety of electronic devices such as solid-state power amplifiers,low-noise amplifiers as well as satellite receivers and transmitters,advanced radar and fiber-optics operating in microwave,submillimeterwave and millimeterwave systems.

A conventional device includes a indium phosphide (InP) substrate; abuffer layer; a quantum well having a first quantum well barrier layer,a channel layer, a second quantum well barrier layer; a donor layer; anda barrier layer (also known as a Schottky layer). The two quantum wellbarrier layers are typically formed of a wide-bandgap semiconductormaterial such as aluminum antimonide (AlSb) or aluminum indium arsenide(AlInAs). The channel layer in the quantum well is formed of anarrow-bandgap semiconductor material such as gallium indium arsenide(GaInAs) or indium arsenide (InAs). The donor layer and the barrierlayer are typically formed of a wide-bandgap material such as AlInAs,AlSb or gallium antimonide (GaSb). The barrier layer may be doped tofunction as both the Schottky barrier layer as well as the donor layer,so that the epitaxial structure does not contain a separate donor layer.

A lattice-matched high electron mobility transistor (HEMT) is a type ofMODFET, where a narrow-bandgap semiconductor material is lattice-matchedto the wide-bandgap semiconductor material. A pseudomorphic highelectron mobility transistor (pHEMT) is another type of MODFET where thenarrow-bandgap semiconductor material is strained in relation to thewide-bandgap semiconductor material.

In these devices, a discontinuity in the energy gaps between the twowide-bandgap semiconductor epilayers layer and the narrow-bandgapsemiconductor channel layer causes electrons to remain in the channellayer. Conduction of electrons therefore takes place in an undopedchannel layer so that the electron mobility is not limited by impurityscattering.

For high-speed and low-noise microwave, submillimeterwave andmillimeterwave applications, manufacturers have been concentrating onproviding HEMTs having a gallium indium arsenide (Ga_(1-x) In_(x) As)channel with high In content because higher electron mobility may beachieved by increasing the In mole fraction. One type ofstate-of-the-art HEMT contains a binary channel wherein x=1 in theformula Ga_(1-x) In_(x) As. It is obvious that, with the high electronmobilities of InAs (values as high as 32,000 cm² -s at room temperaturehave been demonstrated), a binary InAs channel would result in thefastest semiconductor device, provided that one has a suitable materialto form the Schottky for the InAs channel. One possible candidate forincorporating the InAs channel is to use AlSb as the two epilayers.However, among other technical problems, AlSb is chemically unstable. Itwould therefore be desirable to find a chemically more inert materialthan AlSb as the barrier layer.

Another conventional high-speed, low-noise HEMT structure has a tertiarychannel based on the AlInAs/GaInAs material system grown lattice-matchedto the InP substrates. To improve the speed and low-noise operation ofsuch a device, one can incorporate more In to the GaInAs channel (with aformula of Ga₀.47 In₀.53 As if lattice-matched to the InP substrate). Itis also known in the art that incorporating a greater proportion of Inin the Ga_(1-x) In_(x) As channel layer improves the performance of thedevice. However, the amount of In that can be added to the channel layeris limited because an increased proportion of In causes a lattice strainbuildup in the channel layer. Although the strain buildup of the channellayer can be compensated, it is difficult to use AlInAs as astrain-compensating layer because increasing the amount of Al to shrinkthe lattice constant of this material increases the chemical reactivityof the AlInAs and thereby makes the device unreliable. In addition,because AlSb is a binary material, it cannot be used to shrink thelattice constant.

In addition, it is desirable to manufacture a channel with a large sheetcharge density in order to obtain a device with higher current-carryingcapabilities. Increasing the conduction band discontinuity (ΔEc) betweenthe donor layer and the channel layer increases the sheet chargeconcentration in the channel layer. Furthermore, a wide bandgap, largeSchottky material is desirable in order to improve the breakdown andleakage characteristics of the device. Moreover, a high-resistivity,wide bandgap material will be needed in order to improve the turn-offcharacteristics of the device.

SUMMARY OF THE INVENTION

The present invention provides a material system and epitaxial structurefor a field-effect transistor that allows it to incorporate AlP₀.39-ySb₀.61+y in the barrier and/or buffer layers, thereby allowing thefield-effect transistor to provide both low-noise and high-speedcapabilities.

In accordance with a preferred embodiment of the present invention, thenovel material system, which results in a field-effect transistor on asemiconductive support made from an InP substrate, includes a quantumwell and a barrier layer comprising AlP₀.39-y Sb₀.69+y. Preferably, theepitaxial structure includes buffer, donor and barrier layers comprisingAlP₀.39-y Sb₀.61+y and quantum well having a channel made of Ga₀.47-xIn₀.53+x As.

The present invention also encompasses a novel method of making amaterial system for a field-effect transistor. The method steps includeproviding a quantum well over a semiconductive support and providing abarrier layer made from AlP₀.39+y Sb₀.61-y. Preferably, a donor layer isprovided between the barrier layer and the channel layer, the donorlayer is preferably made from doped AlP₀.39+y Sb₀.61-y and the barrierlayer remaining undoped. More preferably, a buffer layer made ofAlP₀.39+y Sb₀.61-y is provided between the quantum well and theseminconductive support and a channel made of Ga₀.47-x In₀.53+x As isprovided in the quantum well.

The epitaxial structure of the present invention results in a reliabledevice suitable for high-speed, low-noise applications because the largeΔEc between AlPSb and GaInAs translates into a larger sheet charge inthe GaInAs channel. In addition, the device provides a flexibleframework for improving the transport and breakdown/leakagecharacteristics and at the same time satisfying the growth requirementof a pseudomorphic structure. The high In mole fraction Ga₀.470-xIn₀.53+x As channel results in a high mobility and low-noise HEMT.Furthermore, the wide bandgap of the AlP₀.39+y Sb₀.61-y results in alarge Schottky barrier. Moreover, because AlP is a more chemicallystable material than AlSb, the material AlP₀.39+y Sb₀.61-y is expectedto be chemically very stable compared to AlSb itself. Finally, with thelarge bandgap, AlP₀.39 Sb₀.61 is an excellent candidate for ahigh-resistivity buffer layer.

The invention itself, together with further objects and attendantadvantages, will best be understood by reference to the followingdetailed description, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a cross-section of a field-effecttransistor structure embodying the present invention; and

FIG. 2 is a schematic diagram of a bandgap lineup of a field-effecttransistor structure embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a novel material system and epitaxialstructure for a high-speed and low-noise HEMT or pHEMT particularlyuseful for microwave, submillimeterwave and millimeterwave applications.

As shown in FIG. 1, an epitaxial structure 10 for a field-effecttransistor in accordance to the present invention includes asemiconductive support 12, a quantum well 14 and a barrier layer 26. Thequantum well 14 includes a first quantum well barrier layer 18, achannel layer 20, a second quantum well barrier layer 22. Preferably,the epitaxial structure 10 also includes a buffer layer 16. Mostpreferably, the epitaxial structure 10 further includes a separate donorlayer 24.

The semiconductive support 12 including a substrate and/or a supportinglayer made from a semiconductor material. For example, thesemiconductive support 12 may be a substrate including InP or asupporting layer including InP.

The buffer layer 16 can include any material suitable for filteringdislocations from the semiconductive support 12 to minimize propagationinto the active layers. The first and second quantum well barrierlayers, 18 and 22, are made from any wide-bandgap semiconductor materialwith a large conduction discontinuity for confining electron flow in thechannel layer 20. For example, the quantum well barrier layers 18 and 22can include a material selected from the group comprising essentially ofAlP₀.39+y Sb₀.61-y and AlIn₀.48+z As₀.52-z. The actual value of zdepends upon the application of the HEMT or pHEMT device. A preferredvalue of z is between about 0 and about 0.52. A more preferred value ofz is between about 0 and 0.12.

The channel layer 20 is made of any narrow-bandgap semiconductormaterial suitable for high mobility electron transport. For example, thechannel layer may be made of Ga₀.47-x In₀.53+x As. Like the compositionof the quantum well barrier layers 18 and 22, the actual value of xdepends upon the application of the HEMT and pHEMT device. A preferredvalue of x is between about 0 and about 0.47. A more preferred value ofx is between about 0.12 and about 0.47.

The barrier layer 24 over the quantum well 14 is made of anywide-bandgap semiconductor material suitable for functioning as aSchottky gate. The preferred wide-bandgap semiconductor for the barrierlayer 24 is AlP₀.39+y Sb₀.61-y. In one preferred embodiment of thepresent invention, the barrier 26 and donor 24 layers includes amaterial having the formula AlP₀.39+y Sb₀.61-y.

In a second preferred embodiment of the present invention, the epitaxialstructure 10 includes a buffer layer 16 including a material having theformula AlP₀.39+y Sb₀.61-y. The more preferred epitaxial structure 10includes buffer 16, donor 24 and barrier 26 layers all made of AlP₀.39+ySb₀.61-y. The most preferred epitaxial structure 10 further includes twoepilayers 18 and 22 made of AlP₀.39+y Sb₀.61-y. For those layers in theepitaxial structure 10 that include AlP₀.39+y Sb₀.61-y, y has a valuebetween about 0 and about 0.39.

The growth of those layers comprising AlP₀.39+y Sb₀.61-y (AlPSb layers)can be achieved by conventional epitaxial growth techniques which supplysources of phosphorous and antimony. For example, an AlPSb layer mad ofAlP₀.39 Sb₀.61 can be grown from a PH₃ gas source equipped with asolid-source antimony. In addition, such AlPSb layer can be grown froman AlP/AlSb superlattice to mimic an AlP₀.39 Sb₀.61 layer. For example,such AlPSb can be grown in accordance with the superlattice growthmethod from a superlattice with a period of about 30 Å and a proper dutycycle, namely about 11.7 Å AlP and about 18.3 Å AlSb, which will mimicthe electronic properties of AlP₀.39 Sb₀.61. Preferably, the AlP/AlSbsuperlattice has a period measurement between about 15 Å and about 50 Å.

The more preferred method of growing the AlPSb layers is growth from aphosphine (PH₃) gas source, and solid-source of antimony. The flow ofphosphine gas can be controlled with a mass flow controller. A thermalcracker at about 800° C. to about 1000° C. cracks phosphine gas into(mainly) dimers before incorporating into the growth front of thelayers. As for the antimony, a well established approach is to generatetetramers and dimers of antimony by thermal means (in an effusion cell),in the same way arsenic is generated in a more conventional solid-sourceMBE growth chamber. High-quality antimonides (GaSb and AlSb) have beenachieved worldwide with or without a thermal cracker. Yet, unlike thearsenic, antimony flux can be shuttered very abruptly. Thus, highquality, abrupt interfaces can be grown.

Conventional epitaxial growth processes can be used to make theepitaxial structure 10 of the present invention. The preferred processesinclude gas-source molecular beam epitaxy (GSMBE), chemical beam epitaxy(CBE), and metalorganic chemical vapor deposition (MOCVD), the latterprocess also known as metalorganic vapor phase epitaxy (MOVPE). The mostpreferred processes are GSMBE and CBE.

Calculations were performed based on the self-consistent ab initio bandstructure methodology of Van de Walle's model solid approach (reference:C. G. Van de Walle, "Band lineups and deformation potentials in themodel-solid theory", Physical Review B, Vol. 39, pages 1871-1881,January 1989) on the material system AlP₀.39 Sb₀.61 and Ga₀.47 In₀.53As. FIG. 2 shows the band lineup between AlP₀.39 Sb₀.61 28 and Ga₀.47In₀.53 As 30, which are lattice matched to InP. Based on thesecalculations, a type I lineup can be expected between AlP₀.39 Sb₀.61/Ga₀.47-x In₀.53+x As (x between 0 and 0.47). In particular, a large ΔEcfrom 1.07 eV (for Ga₀.47 In₀.53 As 30) to 1.21 eV (for InAs) is expectedin this material system. AlP₀.39 Sb₀.61 28 has a very wide bandgap ofabout 1.93 eV.

The epitaxial structure of the present invention results in a reliabledevice suitable for high-speed, low-noise applications because the largeΔEc between AlPSb and GaInAs compared to the conventional ΔEc betweenAlInAs and GaInAs translates into a larger sheet charge in the Ga₀.47-xIn₀.53+x As channel. In addition, the device provides a flexibleframework for allowing the use of strain-compensation. One can add moreindium to the channel (beyond the lattice-matched to InP mole fractionof 0.47 all the way to complete InAs) and at the same time add more P tothe AlPSb barrier. The channel is then under a biaxial compressivestress while the barrier is under a biaxial tensile stress. Increasingthe phosphorous content of the AlPSb barrier produces a compressivestrain in the barrier to compensate for the tensile strain of theIn-rich GaInAs channel. Thus one can improve the transport andbreakdown/leakage characteristics and at the same time take care of thegrowth requirement of a pseudomorphic structure. Also, InAs channel canbe inserted to take advantage of the high electron mobility. The high Inmole fraction Ga₀.47-x In₀.53+x As channel results in a high mobilityand low-noise HEMT. Furthermore, the wide bandgap of the AlP₀.39+ySb₀.61-y results in a large Schottky barrier. Moreover, because AlP is amore chemically stable material than AlSb, the material AlP₀.39+ySb₀.61-y is expected to be chemically very stable compared to AlSbitself. Finally, with the large bandgap, AlP₀.39 Sb₀.61 is an excellentcandidate for a high-resistivity buffer layer.

Of course, it should be understood that a wide range of changes andmodifications can be made to the preferred embodiment described above.It is therefore intended that the foregoing detailed description beregarded as illustrative rather than limiting and that it be understoodthat it is the following claims, including all equivalents, which areintended to define the scope of this invention.

What is claimed is:
 1. A method of making an epitaxial structure for afield-effect transistor, the steps comprising:providing a first quantumwell barrier layer over a semiconductive support, said first quantumwell barrier layer comprising a first wide-bandgap semiconductormaterial; providing a channel layer over the first quantum well barrierlayer, said channel layer comprising a narrow-bandgap semiconductormaterial; providing a second quantum well barrier layer over the channellayer, said second quantum well barrier layer comprising a secondwide-bandgap semiconductor material; providing a barrier layer over thesecond quantum well barrier layer, said donor layer comprising AlP₀.39+ySb₀.61-y, wherein y has a value between about 0 and about 0.39.
 2. Themethod of claim 1 further comprising providing a donor layer between thesecond quantum well barrier layer and the barrier layer, said donorlayer comprising a third wide-bandgap semiconductor material.
 3. Themethod of claim 1 wherein said narrow-bandgap semiconductor material isGa₀.47-x In₀.53+x As, wherein x has a value between about 0 and about0.47.
 4. The method of claim 1 where in said first wide-bandgapsemiconductor material and said second wide-bandgap semiconductormaterial are materials selected from the group consisting essentially ofAlIn₀.48+z As₀.52-z and AlP₀.39+y Sb₀.61-y, wherein z has a valuebetween about 0 and about 0.52 and y has a value between about 0 andabout 0.39.
 5. The method of claim 1 wherein said semiconductive supportcomprises an InP substrate.
 6. The method of claim 2 wherein the thirdwide-bandgap semiconductor material is AlP₀.39+y Sb₀.61-y, wherein yhas, a value between about 0 and about 0.39.
 7. The method of claim 1further comprising the step of providing a buffer layer between thefirst quantum well barrier layer and the semiconductive support, saidbuffer layer comprising a semiconductor material.
 8. The method of claim7 wherein said semiconductor material is AlP₀.39+y Sb₀.61-y, wherein yhas a value between about 0 and about 0.39.
 9. The method of claim 1further comprising the step of doping the barrier layer.
 10. The methodof claim 2 further comprising the step of doping the donor layer. 11.The method of claim 1 wherein:said first quantum well barrier layer isprovided over said semiconductive support by growing said first quantumwell barrier layer over said semiconductive support in a growth chamber;said channel layer is provided over said first quantum well barrierlayer by growing said channel layer over said first quantum well barrierlayer in said growth chamber; said second quantum well barrier layer isprovided over said channel layer by growing said second quantum wellbarrier layer over said channel layer in said growth chamber; saidbarrier layer is provided over said second quantum well barrier layer bygrowing said barrier layer over said second quantum well barrier layerin said growth chamber.
 12. The method of claim 11 wherein said growthchamber is adapted for growth of epitaxial AlPSb from an AlP/AlSbsuperlattice.
 13. The method of claim 11 wherein said growth chamber isadapted for growth of films from a phosphine gas source and asolid-source antimony.
 14. A method of making an epitaxial structure fora field-effect transistor; the steps comprising:providing a buffer layerover a semiconductive support, said buffer layer comprising AlP₀.39+ySb₀.61-y, wherein y has a value between about 0 and about 0.39;providing a first quantum well barrier layer over the buffer layer, saidfirst quantum well barrier layer comprising a first wide-bandgapsemiconductor material; providing a channel layer over the first quantumwell barrier layer, said channel layer comprising a narrow-bandgapsemiconductor material; providing a second quantum well barrier layerover the channel layer, said second quantum well barrier layercomprising a second wide-bandgap semiconductor material; providing abarrier layer over the second quantum well barrier layer, said donorlayer comprising a wide-bandgap semiconductor material.
 15. The methodof claim 14 further comprising the step of providing a donor layerbetween the second quantum well barrier layer and the barrier layer,said donor layer comprising a third wide-bandgap semiconductor material.